1. Field of the Invention
The present invention relates to an optical fiber switch used for an optical fiber communications system or the like. More particularly, it relates to an N.times.2N (N.gtoreq.1) optical fiber switch capable of driving optical fibers so as to mechanically switch a plurality of pairs of optical fiber circuits repeatedly by small electric power. The present invention also relates to an actuator equipped with a self-holding function so that it does not require constant supply of current except at the time of the aforesaid switching for which it momentarily requires a pulse current.
2. Description of the Related Art
There has been proposed an optical fiber switch adapted to directly or indirectly move optical fibers. An optical fiber switch, in which an optical fiber is the only major movable part thereof, is advantageous in that it enables relatively quick switching because it requires an extremely short operating distance and the mass of the movable part is extremely small. On the other hand, it is disadvantageous in that it may damage an optical fiber because it drives a fragile quartz glass optical fiber itself located on the moving side to perform switching.
There has also been a technical task of precisely aligning the optical axes of movable optical fibers with those of fixed optical fibers.
The optical fiber switch disclosed under U.S. Pat. No. 5,434,936 is a 1.times.2 circuit changeover switch which makes use of magnetism to drive a movable optical fiber. A 2.times.2 circuit cross switch disclosed under U.S. Pat. No. 4,834,488 employs a mechanical actuator incorporating a solenoid coil for driving a movable optical fiber. FIGS. 6A through 6D are schematic drawings illustrative of the operation of an optical fiber type optical fiber switch of a 1.times.2 circuit changeover switch, which is comprised of optical fibers, disclosed in U.S. Pat. No. 5,434,936. FIGS. 6A through 6C are top plan views, and FIG. 6D is side sectional view.
Referring to FIGS. 6A through 6D, fixed optical fibers 31 and 32 are adhesively fixed on the bottoms of V grooves 35 and 36, which are respectively formed in the surfaces of a pair of members 33 and 34, in such a manner that the distal end surfaces thereof are aligned. The distal end surfaces of the fixed optical fibers 31 and 32 are positioned so that they are retreated from the left end surfaces of the pair of members 33 and 34.
A movable optical fiber 37 is supported by and fixed to a fixing member 38, the outer surface of the movable optical fiber 37 being coated with a film made of a magnetic material. In a magnetic field of a permanent magnet, the portion coated by the film made of the magnetic material is driven and displaced toward the fixed optical fibers 31 and 32 alternately by changing the magnetic field by a solenoid coil. This causes the movable optical fiber 37 to shuttle in the space to be coupled to the fixed optical fibers 31 and 32 alternately.
In the case of a single-mode optical fiber, a misalignment of 2 .mu.m in optical axis leads to an insertion loss of about 0.86 dB or an optical loss of about 18%. To reduce the insertion loss, therefore, the movable optical fiber 37 must be stopped in contact with the V grooves with its optical axis accurately aligned with the optical axes of the fixed optical fibers 31 and 32. In the optical fiber switch constituted by employing optical fibers, the optical axis of the movable optical fiber 37 can be aligned with the optical axis of the fixed optical fiber 31 only when an appropriate driving force W is applied as indicated by a white arrow in FIG. 6A. However, if the driving force W is insufficient as illustrated in FIG. 6B, the distal end of the movable optical fiber 37 is improperly positioned and cannot reach the V groove 35. Conversely, if the driving force W is excessive, then the portion in the vicinity of the distal end of the movable optical fiber 37 bumps against an edge of the V groove 35 and the distal end of the movable optical fiber 37 is positioned above the V groove 35, thus preventing the optical axis of the movable optical fiber 37 from being aligned with the optical axis of the fixed optical fiber 31. If it is assumed that the permissible displacement of the distal end of the movable optical fiber 37 is 1 .mu.m, it is presumed that accomplishing subtle control of the driving force W to respond to such a minute displacement is hardly possible.
A description will now be made of the 2.times.2 circuit cross switch which employs a mechanical actuator rotated by a solenoid coil to drive a movable optical fiber and which has been disclosed under U.S. Pat. No. 4,834,488. The switch has no alignment means such as a V groove wherein the movable optical fiber is engaged with a fixed optical fiber, presenting doubts about the accuracy in the alignment of the optical axes of the respective optical fibers. Hence, in these conventional examples, the technique for aligning the optical axis of the movable optical fiber with the optical axis of the fixed optical fiber with good reproducibility is imperfect, posing a problem to be solved when constituting an optical fiber switch employing optical fibers.
Optical fiber switches are required to provide improved coupling performance achievable by a reduction in the aforesaid optical insertion loss and to also provide the self-holding feature that enables a coupling position to be secured without the need for the constant supply of current. More specifically, optical fiber switches are required to be able to momentarily flow a pulse current only at the time of circuit switching to hold the coupling between the movable optical fiber and one of the fixed optical fibers. The 2.times.2 circuit cross switch disclosed under U.S. Pat. No. 4,834,488 requires constant supply of current. The optical fiber type optical fiber switch of the 1.times.2 circuit changeover switch constructed using optical fibers (disclosed in U.S. Pat. No. 5,434,936) is the self-holding type; however, the movable optical fiber 37 having its outer surface coated with the film made of a magnetic material is inherently susceptible to magnetic force. Hence, this switch is presumed to have a difficulty in the reliability of the self-holding performance if it is subjected to an external magnetic field or impact.
Because of the reasons set forth above, it is difficult to implement an optical fiber switch, which is capable of switching a plurality of optical fibers at the same time, by utilizing the structural principles of the optical fiber switches of the prior art examples described above. To achieve an optical fiber switch that permits switching of a plurality of optical fibers at the same time, it is required to minimize the insertion losses of all pairs by accurately aligning the optical axes of all matching pairs of movable optical fibers and fixed optical fibers. It is also required to provide reliable self-holding feature so as to achieve higher reliability of switching operation.